1. Field of the Invention
The present invention generally relates to a voltage controlled oscillator and a RF device. The present invention specifically relates to a balanced oscillator consisting of a frequency elective circuit and a balanced active circuit coupled with the frequency selective circuit.
2. Description of the Related Art
A prior art balanced oscillator disclosed in EP-A-0 133 799 has a balanced frequency selective circuit in the form of a tuning circuit and a balanced active circuit in the form of a feedback amplifier pair coupled with the tuning circuit. The balanced tuning circuit has a main electrode having a neutral ground terminal and providing a distributed inductance, the main electrode having balanced terminals. An auxiliary electrode is disposed in confronting relation to the main electrode with a dielectric interposed therebetween, and has opposite ends serving as ground terminals.
It is a disadvantage of the known balanced oscillator that it has a complex construction with a multiplicity of grounding terminals and electrodes. Such a construction is difficult to integrate on a limited chip area.
Accordingly it is an object of the present invention to provide a balanced oscillator, which can be easily and cost effectively integrated on a limited chip area and which has a minimum number of components.
Thereto the balanced oscillator is according to the present invention characterized in that the frequency selective circuit comprises a short circuited quarter-wave paired line.
It is an advantage of the oscillator according to the present invention that such a short circuited quarter-wave paired line also is a balanced circuit, which leads to a fully balanced oscillator. Such a fully balanced oscillator is easy to integrate in a reproducible and accurate manner. In addition its geometry is such that it only has to have one supply voltage (or ground terminal) such that its construction is simplified. Furthermore the short circuited quarter-wave line AC-isolates the supply of the oscillator from the active circuit, thus improving the frequency stability and reliable operation of the oscillator according to the invention.
In one embodiment of the balanced oscillator according to the present invention the short circuited quarter-wave paired line is essentially U-shaped, which reduces the necessary chip area even further.
A further embodiment of the oscillator according to the present invention is characterized in that the short circuited quarter-wave paired line is coplanar. Advantageously this results in the absence of a voluminous ground plate in the frequency selective circuit of the present oscillator. In addition the coplanar nature of the paired line results in a diminished electromagnetic coupling to neighboring circuits.
A still further embodiment of the oscillator according to the present invention is characterized in that the coplanar short circuited quarter-wave paired line is integrated on a dielectric isolator or isolator, such as glass, alumina, silicon dioxide or any other insulator material used in the semiconductor industry. In particular glass shows low losses, and a well achievable coplanarity thereof does not result in a significant electromagnetic coupling to any circuit present at the other side of the isolator or dielectric isolator.
Another embodiment of the oscillator according to the present invention is characterized in that the balanced active circuit comprises a differential amplifier, which shows accurate reproducible properties and furthermore consumes low power for supplying a negative resistance while functioning as an active circuit in the oscillator. In particular, a cross-coupled feedback amplifier is preferred because of its additional reduced number of components thereof.
Easy tuning with a still further reduced number of components is achieved in still another embodiment of the balanced oscillator according to the present invention, which is characterized in that the tuning circuit in particular comprises at least one of the following means: (i) varactor (variable capacitor) means, (ii) oscillator supply voltage varying means, (iii) differential amplifier tail current varying means, and (iv) means for trimming the length of the short circuited quarter-wave paired line.